1. Field of the Invention
The present invention relates to a laminated balun transformer used as a balanced-to-unbalanced transformer for an integrated circuit (IC) in a mobile phone, for an antenna for a television receiver, or the like, and particularly relates to a laminated balun transformer having, for example, an impedance transformation ratio of one to four.
2. Description of the Related Art
As illustrated in an equivalent circuit diagram of FIG. 19, a one-to-four type balun transformer includes a first transformer 100 and a second transformer 200. By connecting coils 101 and 102 included in the first transformer 100 to coils 201 and 202 included in the second transformer 200 as illustrated in FIG. 19, the ratio of impedance on the side of an unbalanced terminal 300 to impedance on the side of balanced terminals 301 and 302 is made to be one to four.
Examples of conventional laminated balun transformers having a circuit configuration of this type are disclosed in Japanese Unexamined Patent Application Publication No. 04-206905 and Japanese Unexamined Patent Application Publication No. 06-120428.
These laminated balun transformers are characterized in that they are constructed by stacking magnetic sheets, non-magnetic sheets, and conductive patterns for forming coils of the first and second transformers 100 and 200, and thus are compact in size.
When the conventional laminated balun transformer described above is used for an IC in a mobile phone or the like, high-frequency signals are processed therein. This causes parasitic inductance to occur at many points in inter-transformer connection lines which connect the transformers 100 and 200. This parasitic inductance will be a major factor that will degrade balance characteristics (i.e., amplitude balance and phase balance) between differential signals input to and output from the balanced terminals 301 and 302. However, in the conventional laminated balun transformer described above, since no effective mechanism is provided to deal with such parasitic inductance, degradation of balance characteristics cannot be avoided.